1. Field of the Invention
This invention relates to the field of aerospace, and generally to the deployment of small satellites from high-energy orbits to lower-energy orbits, via aerobraking.
2. Description of Related Art
Satellites, because of their unobstructed fields of view of large areas of the earth, are often the preferred technical solutions to a variety of communications and monitoring problems as well as space and earth science applications. The high cost of satellite deployment, however, often precludes their use from a cost-efficiency viewpoint. In order to distribute the high costs of deployment among a large number of applications or users, the majority of deployed satellites are those that handle a multitude of tasks, or a multitude of customers for the same task. To minimize the loss of available access time to or from the satellite from or to the location on earth being serviced by a satellite, many satellites are placed in geosynchronous orbit. A geosynchronous orbit tracks the revolution of the earth, so that the satellite appears to be fixed over the same area of the earth, thus providing continual access to that area. Due to the physics involved, a geosynchronous orbit is approximately 36,000 kilometers above the earth. To provide reliable communications over this long distance, a geosynchronous satellite requires highly sensitive receivers and/or highly powerful transmitters. Because of the aforementioned economic and technical requirements, geosynchronous satellites generally weigh a ton or more, and cost (in 1999) hundreds of millions of dollars to deploy to the selected geosynchronous orbit.
As contrast to large geosynchronous satellites, the use of small satellites at low-earth orbit (LEO) are becoming increasingly common. U.S. Pat. No. 6,128,469, xe2x80x9cSATELLITE COMMUNICATION SYSTEM WITH A SWEEPING HIGH-GAIN ANTENNAxe2x80x9d, issued Oct. 3, 2000 to Fleeter et al.; U.S. Pat. No. 6,396,819, xe2x80x9cLOW-COST SATELLITE COMMUNICATION SYSTEMxe2x80x9d, issued May 28, 2002 to Fleeter et al.; U.S. Pat. No. 6,317,029, xe2x80x9cIN SITU REMOTE SENSINGxe2x80x9d, issued Nov. 13, 2001 to Richard Fleeter; and U.S. Pat. No. 6,296,205, xe2x80x9cRF INSPECTION SATELLITExe2x80x9d, issued Oct. 2, 2001 to Hanson et at. illustrate the use of low cost satellites for a variety of applications, and are incorporated by reference herein. Low-earth orbits arc typically hundreds of miles above the earth, rather than thousands of miles. Because of their order of magnitude closer proximity to earth, satellites in low-earth orbit require significantly less communicating and monitoring power and sensitivity than the satellites in geosynchronous orbit. Because they are not stationary above any location on the earth, multiple satellites in low-earth orbit are required to provide continuous coverage of a particular area on earth. Because multiple satellites are required in low-earth orbit to provide continuous coverage, a low-earth orbit satellite system is particularly well suited to applications that employ low cost satellites. As advances continue to be made in electronic circuit density and efficiency, the number of communication and monitoring applications that can be embodied in small, low cost satellites continues to increase.
Deployment of a small, less than five hundred pound, satellite into low-earth orbit typically costs, in 1999 dollars, between ten and twenty-five million dollars. Because a plurality of satellites is required to provide continuous coverage of an area, the overall cost of deploying constellations of low-earth orbit satellites can often amount to hundreds of millions of dollars.
It is an object of this invention to provide a lower cost means for deploying a satellite into low-earth orbit. It is a further object of this invention to provide a method for economically brokering the deployment of a satellite into low-earth orbit. It is a further object of this invention to provide an orbit-transfer vehicle to effectively deploy small satellite systems to low-earth orbit.
A launch of geosynchronous satellites typically includes one or two large, multi-ton, satellites that are deployed at the geosynchronous altitude of 36,000 kilometers via a large multi-stage rocket system, such as an Ariane system. Typically, after allocating the available space and weight capabilities of the rocket system to the primary payload of the one or two large satellites, some excess space and weight allocation remains. For example, if an Ariane launch vehicle can accommodate four tons, and the primary payload satellites are 1.5 and 2 tons each, the launch vehicle has an excess capacity of a half ton. Because the marginal cost of adding one or two small satellites is minimal, this excess space or weight capacity can be brokered for the deployment of small satellites at substantially less cost than the primary payload, often less than a quarter of the cost per pound charged to the primary payload satellites.
The expressed objects of this invention, and others, are achieved by providing a means of utilizing the excess space and weight capacity that is typical of a launch of large satellites to high-energy orbits, such as a geosynchronous orbit, to deploy small satellites at a substantially lower-energy orbit, such as a low-earth orbit. In a preferred embodiment, an orbit-transfer vehicle provides the navigation, propulsion, and control systems required to transport a payload satellite from a geosynchronous-transfer orbit (GTO) to a predetermined low-earth orbit (LEO). Depending upon the particular configuration, upon achieving the low-earth orbit, the orbit transfer vehicle either releases the payload satellite, or remains attached to the payload satellite to provide support services, such as power, communications, and navigation, to the payload satellite. To reduce the fuel requirements for this deployment via the orbit-transfer vehicle, a preferred embodiment includes aerobraking to bring the satellite into a low-earth orbit. The aerobraking is preferably performed at a nominal altitude of 150 km above the earth, where the atmosphere is dense enough to allow for a reasonably sized drogue device, yet rare enough to avoid the need for special purpose heat-shielding materials. In a preferred embodiment of this method of deployment, the provider of the orbit-transfer vehicle identifies and secures available excess capacity on geosynchronous-transfer launch vehicles, and allocates the excess capacity to the satellites requiring low-earth orbit deployment, thereby providing a deployment means that is virtually transparent to the purchaser of this deployment service.